The object of the work proposed is to develop an advanced above knee (AK) prosthesis which is more natural to walk with, requires less effort, and increases capabilities for walking on slopes and at higher gait speeds. Currently available knee prostheses do not actively adapt to activities of the amputee. The proposed system would modify the behavior of the prosthesis in response to several potential parameters: position, velocity or acceleration of the joint, loads in the foot and leg, and/or control inputs from the amputee. It is proposed to accomplish this by designing a small electrohydraulic control valve, a spring/damper mechanism and a microprocessor based controller. The goal is to allow above-knee amputees to walk and run more smoothly and stably and adapt easier to variations in cadence and activity, e.g., uphill and downhill walking and sports activities. Two important developments are necessary for the realization of a practical adaptively controlled above knee prosthesis. The first is the development of a practical variable impedance mechanism for the knee which can be actively controlled, and the second is the development of an adaptive controller which will command the mechanism to exhibit appropriate characteristics. These characteristics should range from free unrestricted motion to high viscous damping or to high energy-storing stiffness.